250 DPS for you just standing there pressing 1 button every 13-14 seconds is extreme high. And sp cost after stacking this 3 times is very low - from your own calculations (40460dmg at 156sec) 1 cast is about 3500dmg on average, it is much more sp effcient than any other damage spell in game (with possible exceptions of fw and ice storm, but they are not stacking from different casters, provide much lower dps on bosses and suffer much more from resists/immunities). Even with Sirgog's estimations of 2800 damage/cast with only basic gear it is incredibly efficient. And this is for cleric, not for FVS, who has 30% damage more from PrE.
About healing reducing non-DP dps - that is right, but you can take WF FVS to your raid as dps, not a healer and according to my example, he should be as good dpser as similary geared barb. And to make things funnier, with lower-end gear fvs will be better than pure melee, because gear impacts melee dps much more than DP and archon dps.